1. Field of the Invention
The present invention relates to the apparatus for manufacturing flat glass wherein the glass is formed while being supported on a surface of a pool of molten metal following its delivery thereto along an inclined support member which contacts the molten metal and provides continuous support for the glass throughout its delivery until it is supported on the molten metal. More particularly, this invention relates to a unique combination of elements comprising suitable means for supporting the molten glass throughout its delivery onto the pool of molten metal.
2. Brief Description of the Prior Art
It is known that molten glass can be delivered onto molten metal and formed into a continuous ribbon or sheet of glass according to the teachings of Heal, U.S. Pat. No. 710,357; of Hitchcock, U.S. Pat. No. 789,911; of Pilkington, U.S. Pat. No. 3,083,501 and U.S. Pat. No. 3,220,816; and of Edge and Kunkle, U.S. Pat. No. 3,843,346. In all of the prior art, molten glass is delivered over some rigid element, usually a refractory member, onto the molten metal. In all of the mentioned patents, except those of Pilkington, the molten glass is in contact with its supporting refractory element from the time it flows from a pool of molten glass in a tank or furnace until it is in contact with the molten metal upon which it is to be supported and formed. In the practice disclosed by Pilkington, the molten glass is delivered through a long, narrow canal and over a lip from which the molten glass falls onto the molten metal and spreads outwardly on the molten metal.
Several patents and publications illustrate devices for delivering molten glass from a tank or furnace onto a pool of molten metal containing a forming chamber wherein the device provides a supporting surface for the molten glass throughout its flow from the tank or furnace into the forming chamber until it is supported by the molten metal in the forming chamber. Several of these patents and publications disclose that the glass supporting member for delivering the stream or layer of molten glass onto the pool of molten metal may be inclined plane or other downwardly sloped member. Representative of the disclosures of such devices are U.S. Pat. No. 3,305,339 to Plumat, U.S. Pat. No. 3,442,636 to Kita et al and a paper of L. A. B. Pilkington delivered in 1969 (L. A. B. Pilkington, Processing Royal Society, London, Volume 314A, pages 1-25, 1969).
The device of Plumat employs an adjustable inclined plane having glass heaters mounted in it. The supporting inclined plane of Plumat consists of a metal which is heat-resistant and neutral with respect to the molten glass. Molybdenum and tungsten are suggested metals. Plumat also indicates that other refractory metals might be employed and that such metals could be covered with a sheet of noble metal, such as platinum, in the event that such metals are not neutral with respect to molten glass. The use of materials such as those suggested by Plumat would likely result in significant problems for the metals suggested by Plumat, including molybdenum, tungsten and platinum which are suitable for contact with molten glass, are not well suited for contact with molten metals, such as tin, which are suited for supporting molten glass to form it into flat glass.
The device suggested by Kita et al provides a glass supporting member that is fixed in place with respect to the glass forming chamber and is formed of a refractory material resistant to corrosion by the glass or of a material on which molten glass slides readily, such as graphite. Graphite is, of course, compatible with molten metal such as tin. Difficulties could be expected when employing a monolithic graphite piece as a delivery support for molten glass from a furnace to a forming chamber because of the fact that the glassmaking furnace is generally maintained under oxidizing conditions which would be detrimental to the life of graphite. Further, the use of graphite at a location at which continuous contact with molten glass occurs would likely result in the discoloration of glasses containing highly-oxidized colorants or the introduction of fine bubbles into glass of practically any composition.
Pilkington, at pages 13 and 14 of the mentioned article, describes the use of a refractory spout which dips into a pool of tin in a forming chamber. Molten glass is poured over the refractory spout and onto the surface of the pool of molten tin. Pilkington states that molten glass is chemically active and dissolves all refractory materials to some extent and that the rate of wear of the refractory surface is greatly accelerated at the tin-glass-refractory interface where the spout is dipped into the tin. Pilkington further indicates that glass that has been in contact with the refractory spout becomes contaminated causing optical defects in bottom surfaces of finished glass. Eventually, as described in Pilkington in that same article, the problem associated with refractory glass reaction in the vicinity of molten tin refractory glass interface was avoided by simply shortening the spout so that it was not in contact with molten tin. This eliminated the interfacial region at which the undesirable reactions were believed to predominately occur. As reflected in the earlier-mentioned patents of Pilkington, this feature of eliminating the continuity of support for molten glass and, thus, eliminating the refractory molten metal interface constituted the basic concept of the inventions of Pilkington relating to float processes.
In the practice of a float process such as that described by Pilkington, molten glass falls freely from a refractory support onto the surface of a pool of molten metal, such as tin, spaced from a refractory support. The glass then flows rearwardly under the refractory support and outwardly from it on a surface of the pool of molten metal to form a layer or body of molten glass which can be drawn along the surface to form a continuous sheet of glass. The flow of molten glass beneath such a refractory spout or support is called a wetback flow and is directed for a short distance in a direction opposite to the general movement or advance of glass through the system. This opposite or rearward flow moves into the vicinity of, or into contact with, a back perimeter wall of the forming chamber known as a wetback. From time to time defects are encountered in the glass formed in such a process which defects have been found to be related to contamination conditions, undesirable stagnation of glass flow beneath the glass delivery spout, ingress of air through the wetback and devitrification of glass in that region of a forming chamber. Another problem solved by the present invention is that by the provision for a readily replaceable refractory piece, short circuiting glass flows past worn jamb blocks and worn lips are easily avoided. While such problems of contamination and stagnation in a rearward flow region do not exist in a process such as that described by Edge and Kunkle, there exists in the United States today and throughout the world a substantial number of glass forming facilities built according to the teachings of Pilkington prior to the development of the process described and claimed by Edge and Kunkle. Due to the substantial structural differences between the delivery facilities of such existing units and the structural features of a glass forming chamber and its associated molten glass delivery facilities such as described by Edge and Kunkle, there has been presented an incentive to develop some means for modifying existing float forming facilities employing free-fall molten glass delivery systems to avoid the problems associated with wetback or rearward flows. Modifications have been desired which would be more simple and more expedient than completely rebuilding the delivery facilities of such units and altering the relative elevations of the glass forming chambers and the glass tanks or furnaces to provide the kind of direct delivery system described by Edge and Kunkle.
The invention of the applicants comprises an apparatus for accomplishing such a purpose.